In the cell search, the user equipment UE acquires a cell ID in addition to a received sub-frame and radio frame timings in the downlink. The cell search process must also be performed periodically in order to update the cell to be connected and to find a candidate cell for handover. The cell ID corresponds to a cell-specific scrambling code, which is necessary to randomize other-cell interference in cellular system with multi-cell configuration. The cell ID composition and the general cell search procedure are described herein after.
According to 3G LTE specifications, downlink transmission is organized into radio frames with a duration of 10 ms. Each radio frame consists of 10 sub-frames, each with two consecutive 0.5 ms slots. Data are mapped on a time-frequency resource grid consisting of elementary units called resource elements (RE). They are uniquely identified by the transmit antenna, the sub-carrier position and the OFDM symbol index within a radio frame.
A dedicated synchronization channel (SCH) is specified in 3G LTE for transmitting two synchronization signals, the primary (PSS) and the secondary (SSS). Within the synchronization channel, both synchronization sequences are mapped on 62 subcarriers located symmetrically around the DC-carrier. They are transmitted within the last two OFDM symbols of the first and sixth sub-frame (sub-frame index 0 and 5), i.e. every 5 ms.
The PSS signal consists of three length-62 Zadoff-Chu sequences in frequency domain which are orthogonal to each other. Each sequence typically corresponds to a sector identity Ns=0, 1 or 2 within a group of three sectors (physical cell). The value of Ns is also referred to as CID2 (Cell ID 2) in the LTE specification.
The SSS signal consists of a frequency-domain sequence of OFDM symbols d(n) with the same length as the PSS, which is an interleaved concatenation of the two length-31 scrambled binary sequences s0(n) and s1(n). In order to distinguish between different sector groups (physical cells), s0(n) and s1(n) depend on a pair of integers m0 and m1, which are unique for each group-ID Ng (from 0 to 167). The value of Ng is also referred to as CID1 (Cell ID 1) in the LTE specification.
The concatenated sequences are scrambled again with one of the sequences c0(n) and c1(n), which are cyclic shifted versions of the length-31 binary sequence {tilde over (c)}(n).
The binary sequence {tilde over (c)}(n) is a scrambling sequence, which depends on the PSS information (CID2). The fact that it is used for descrambling doesn't make the detection “coherent” because descrambling is perhaps needed for detection but is not detection.
Indeed, the detection takes some information from the PSS detection (the PSS timing, the CID2) but the SSS detection doesn't use a channel estimate from an external source and is as a consequence “non-coherent”.
The shift value is depending on the sector-ID Ns, while a constant shift of 3 samples holds between c0(n) and c1(n).
Further, a pair of scrambling sequences z1m0 (n) and z1m1 (n) (cyclic shifted versions of sequence {tilde over (z)}(n)), which are multiplied with the odd entries of the SSS.
{tilde over (Z)}(n) is a known, fixed scrambling sequence corresponding to a fixed sequence of 31 0's and 1's defined in the LTE standard, which does not depend on Ng or Nc. In order to form z1m0(n)/z1m1(n), this scrambling sequence is cyclically shifted by (m0/m1) modulo 8, so there are 8 possible shifts for the original scrambling sequence.
In order to enable the detection of beginning of radio frame, the SSS signal is different for each sub-frame index (0 or 5), thus the final SSS sequence d(n) is described in the standard of LTE 36211 and is given by:d(2n)=S0m0(n)c0(n) in sub-frame 0,d(2n)=S1m1(n)c0(n) in sub-frame 5.andd(2n+1)=S1m1(n)c1(n)z1m0(n) in sub-frame 0,d(2n+1)=S0m0(n)c1(n)z1m1(n) in sub-frame 5.
The set of (m0, m1) pairs have 1-1 correspondence with the encoded Ns so that decoding this pair permits decoding Ns.
As d(n) is mapped to real valued BPSK constellation, time domain symmetry always holds for the SSS signal. The overall cell-ID Nc (from 0 to 503) is equal to Nc=3Ng+Ns is thus defined by the sector and group identities Ns and Ng. The value of Nc is also referred to as CID (cell ID) in the LTE specification. As a consequence, the CID depends on CID1 and CID2. When the CID2 is known (given by the PSS information), the CID detection corresponds to the cell group ID (CID1) detection.
In 3G LTE system, initial cell search procedure comprises two steps using PSS and SSS as shown in FIG. 1. First, OFDM symbol timing and physical-layer ID are detected by PSS in time domain. Second, radio frame timing and cell group ID are detected by SSS in frequency domain. In general, coherent detection using estimated channel frequency response (CFR) at PSS is used for SSS detection. Nevertheless, it is possible to use both coherent and non-coherent detection for detecting the SSS and thus identifying the cell ID to which the user equipment is connected.
Some problem remains when searching for cell ID.
For example, when performing cell-search in 3G LTE with the SSS (Secondary Synchronization Signal), and when there exists very strong interference on the desired cell from a neighbor cell having the same Ng (modulated on the PSS) and the same frame timing (e.g. in TD-LTE), the detection performance of typical coherent detector suffers from dramatic degradation.
For instance, the performance of the Zero-Force (hereafter ZF) Correlation, Complex-Conjugate Correlation and Maximum-Likelihood (hereafter ML) is severely degraded.
The detection performance becomes unacceptable because the CFR (channel frequency response) needed for coherent detection is estimated by de-correlating the input samples FFT with the PSS information sequence, and a distortion of the CFR is obtained when estimating it from the interfered PSS signal.
The issue with interference is as follows: two cells transmit the exact same PSS signal. And this signal is received in the user equipment (UE) in the exact same time if the distance between the two cells and the user equipment is the same. In this case, the user equipment can not separate the PSS part coming from each of the two cells, so as to estimate the channel from each cell separately. As a consequence, the channel estimate from the PSS can not be used in the general case.
The problem becomes critical when doing initial cell-search and there is no information about the cell ID of the interfering cell, so that subtraction of interference is not possible.
Integration over multiple SSS OFDM symbols improves SNR performance but has no impact on detection when averaging the neighbor cell distortion effect in static channel conditions.
Thus, there exists a need for a method for performing cell search when strong interferences are present in the signal received by the user equipment. More particularly, there exists a need for a method allowing a cell search in case of a low power ratio between the desired carrier power and total interference power from cells transmitting the exact same PSS signal.
However, coherent correlation methods are desirable in case of channel dispersion or inaccurate timing estimate of the SSS position. This is because the channel estimation and de-correlation negates the effect of channel dispersion or timing inaccuracy. Such dispersion causes performance degradation in simple non-coherent integration methods that correlate the input signal directly with the SSS OFDM symbol hypotheses.
Current methods are based on:    a) Coherent integration (where the channel is estimated from the PSS), which suffers from severe degradation in case of strong interference,    b) Non-coherent integration without any search of the received channel, which suffers from non-immunity to channel response variation across frequency,    c) Coherent correlation when the interference is subtracted from the estimated channel in the SSS correlation. This method is good when the interface cell ID is known but not in an initial cell-search situation.